This invention is in the field of video display systems, and is more specifically directed to the sampling of analog input video signals for display on a digital video display.
As is well-known in the industry, many video display systems now operate in the digital domain, with the brightness and color of each picture element (pixel) in the displayed image controlled according to a digital value. As is also known in the industry and in the art, analog video signals are still prevalent to a large degree, especially as used in the communication and display of television content. In addition, many personal computers still present their video output in analog form. While digital video interfaces (DVI) are known, the additional cost associated with DVI video, and its relatively recent deployment, has resulted in analog video still being widely used, even in new systems.
The digital display of images communicated by analog video signals thus requires the conversion of the image data from the analog domain to the digital domain. And, of course, this analog-to-digital conversion requires the sampling of the analog signal to derive the digital representation. Accurate and faithful digitization of the analog video signal requires accurate and faithful sampling of that analog signal. In an ideal situation, this sampling is straightforward, considering that conventional analog video signals are represented by a sequence of voltage levels (e.g., associated with the luminance and chrominance components), each associated with a pixel on the display and having a duration of a period of the pixel rate (i.e., the pixel period), and each voltage level being relatively constant over the pixel period. Ideally, sampling of the analog video signal at the pixel rate will result in accurate digitization of the analog signal, at the source pixel rate.
However, the analog video signal waveform for each pixel is seldom ideal. FIG. 1 illustrates an example of an analog video signal for given pixel, in which transitions to the analog level are made both prior to and after that pixel, from and to different analog voltage levels. A signal such as that shown in FIG. 1 will be repeated for each pixel in the display field or display frame, with the level of each pixel corresponding to its brightness or color value. As shown in FIG. 1, the leading edge of the signal for this pixel has substantial ringing during its settling time shown as ttr. After this settling time, the signal remains relative stable (during time to). But near the end of the pixel period, the trailing edge of this pixel signal also exhibits ringing during time ttf. It is therefore apparent that it will be more accurate to sample this pixel signal during time to, rather than during times ttr and ttf. Not only will the sampled value tend to be incorrect if acquired during one of times ttr and ttf, but because the ringing may vary in amplitude, duration, and phase from pixel to pixel, the sampled values for the same pixel will vary from field to field and frame to frame, even if the color or brightness value of the pixel remains constant. The resulting displayed image will be of poor fidelity if samples are routinely acquired in these unstable times ttr and ttf.
Of course, reducing the amplitude and duration of the ringing at transitions of the signal will increase the fraction of the pixel period during which accurate samples may be taken, and will also reduce the error resulting from sample points set or drifting within the transition and settling portions of the pixel period. However, the pixel rate required for the communication of display image fields or frames at the resolution of modern high resolution displays requires extremely fast switching times in the analog video signal for a given frame rate. These fast switching times not only reduce the pixel period within which the sampling must reliably take place, but also increase the amplitude of ringing. As such, it is much more difficult to accurately sample and digitize analog video signals for higher resolution displays.
Even at such high pixel rates, it is often possible to detect a stable portion of each pixel period in which accurate samples can be acquired. However, modern display systems are called upon to display images at various resolutions and frame rates, either as may be determined by a specific application, or as may be selected by the system user. Modern displays must therefore be able to sample at various pixel frequencies, and at various points within each pixel period, in order to handle this wide range of resolutions and frame rates.
Phase optimization techniques for determining a good sample point within each pixel period are known in the art. One common approach varies the sampling point within the pixel period from frame-to-frame, and compares the pixel values for successive frames to identify the optimum one of the various sample points. This approach necessarily involves degrading of the image as displayed, because the optimum sample point cannot be discerned without a degraded image against which to compare the optimum sample point. This degrading of the displayed image discourages adjustment of the sample phase during actual operation. In addition, this approach is excessively memory-intensive, because the pixel results from each of the sampled pixels must be stored for comparison with the next frame. Considering that many modern displays are 1600 by 1200 pixels in size, the memory requirements for a single sample phase can exceed two million bytes or words. While the memory requirements for this approach can be reduced by reducing the number of sample phases attempted, this reduction in possible sample phases will also limit and slow the optimization.
Another approach involves sampling the pixel values at various sample phases within a single frame. In this way, the accuracy of the sample for each pixel can be determined, using the pixel value for that pixel and that frame as a reference. However, this approach requires the sample rate to be a large multiple of the pixel rate, in order to accurately sample the same pixel level multiple times within each pixel period. For example, if thirty-two possible sample phases are to be attempted for a 1600 by 1200 display with a refresh rate of 60 Hz, the sample rate would be on the order of 4.4 GHz, which is of course a prohibitively high sample rate for modern technology.